1. Field of the Invention
The present invention is directed to a multilayer chemically-milled circuit assembly. It is more particularly directed to a method of manufacturing a multi-layer circuit assembly in a continuous process. During the course of the process, an interim step includes the application of the conductive patterns to both sides of a flexible film carrier prior to a final etching step which fully develops the circuitry.
2. Description of the Related Arts
Double-sided (two conductor layers) and Multi-layer printed circuit boards (more than two conductor layers), as the terms have come to be known in the art, are used extensively today in an effort to increase the interconnect density required by complex microelectronic systems. The most commonly used printed circuit assembly is a "double-sided" assembly having top and bottom metallic layers separated by a middle electrically insulating layer usually consisting of fiberglass-reinforced epoxy or any number of other polymer dielectric materials. The well known two-layer concept has been successfully modified by bonding multiple double sided printed wiring boards with partially reacted glass-epoxy insulating layers. Current art encompasses many multilayer printed wiring board process variations. Some of these variations involve the interconnecting vias or plated-thru holes while yet other variations involve the sequence of outer layer photolithography and etching. Regardless of the specific multilayer printed wiring board process, it is generally acknowledged that multiple layers exact cost and quality penalties but provide the necessary advantage of increasing functionality and component density.
Another type of two conductor layer circuit board may be considered in which both the top and bottom layers have conductor patterns formed on a middle metallic layer which are connected through the use of "air bridges" by selectively etching portions of the middle layer. The bottom layer, in turn, is typically affixed to a base material such as, for example, an epoxy glass substrate, generally referred to as FR4, or an aluminum base plate having a dielectric layer to prevent shorting or any two or three dimensional solid material with an electrically insulating surface.
A process for manufacturing such two-layer chemically-milled printed circuit boards is shown in substantial detail in U.S. Pat. No. 4,404,059 issued to Livshits et al. Livshits teaches that conductor patterns may be formed into a double-sided printed circuit board through the use of an additive procedure referred to in the art as "RITM." As disclosed, conductor metal is electroplated onto opposed major surfaces of a substantially planar metal substrate through respective protective masks which correspond to the desired conductor pattern. The protective masks are typically photoresists which are applied by known two-side photolithography over the metal substrate. The conductor pattern on the first major surface includes bridging elements having enlarged ends and a constricting portion therebetween. The conductor pattern on the opposed major surface includes elements oriented transversely to the bridging elements.
After the outer conductors are electroplated on the middle metallic layer, the protected masks are removed and following a pre-etching step, an adhesive layer comprising an insulated material is secured to one major surface of the plated substrate such that it becomes embedded in one major surface of the adhesive layer. The other major surface of the adhesive layer is thereafter secured to a base. The entire substrate is then immersed into an etchant for a sufficient period of time to remove exposed portions of the substrate below the constricted portions of the bridging elements throughout the entire thickness of the substrate.
Livshits et. al. suggests that a double-sided printed (two conductor layers) circuit board may also be formed by selectively removing conductor metal pre-applied over the entire surface of the substrate with the advantage that pre-fabricated bimetallic laminates prepared by metallurgical cladding techniques may be used. Livshits cautions, however, that the resulting conductors will suffer from the irregularity of edges and low produceability of shape. Accordingly, the additive technique discussed above is proclaimed as more efficient from the standpoint of conductor metal consumption as well as the attainment of higher density of the conducting pattern of the panel.
Another method of manufacturing two-layer chemically-milled circuit boards is shown in U.S. Pat. No. 3,801,388, issued to Akiyama et al. Akiyama et al. is similar to Livshits et al. except that the process starts with a tri-metel clad structure in which a metal such as copper is clad on both sides with a metal having substantially different etch characteristics such as nickel. The tri-metal layer is coated with a resist coating which is exposed and developed. The nickel is defined into the eventual circuit traces by using the photoresist as an etch resist. The remainder of the process is the same as that already described.
The Livshits et al and the Akyiama patents may be viewed as "additive" and "subtractive" variants of the same basic chemically-milled circuit process in that both processes result in substantially the same two-layer chemically milled interconnect construction. The resulting two conductor layer circuit structures are dominated by air bridge conductor cross-over and solid pedestals which are quite different in appearance to traditional printed circuit board features.
Among the difficulties with both the two-layer laminated printed circuit board and two conductor layer chemically-milled processes described above are the cost and size of the resulting circuit board. The conductor patterns are fixed to a rigid substrate (circuit board) prior to etching. Use of the rigid board has required a batch that both increased the cost and size of the resultant circuit. With increased interconnect density requirements (higher electrical functionality in a smaller volume) such two-layer chemically-milled circuit approaches have only limited utility. A need exists for low cost, flexible multi-layer (more that two) chemically milled circuitry. Since both layers of this construction are necessary for X and Y conductor routing, there is no provision in either the Akyiama or the Livshits constructions for EMI or RF shielding, nor higher interconnect densities required on fine pitch devices. There is also a practical limitation of the outer metal thickness which may be used on the two-layer construction. Thinner outer metal layers are required for finer resolution while precluding high currents due to inherent conductor resistive heating. The common requirement, especially in automotive electronic packaging, of mixed power/logic functionality limits the practical utility of two-layer chemically-milled circuit approaches
Some attempt to manufacture flexible circuits are shown in U.S. Pat. No. 4,659,425, issued to Eggers et al. Eggers et al. shows a continuous method of manufacturing a circuit assembly. A resin is applied to the surface of a metal foil. The resin coated foil is contacted with a reinforcing cloth in a double belt press. The method produces a double sided circuit. The double belt process is relatively slow and requires a large amount of floor space. It is primarily directed to laminating a reinforcing cloth to a film and does not enable the use of extruded film between the metal foils.
There are rumerous other known flexible circuit processes. The majority involve the use of a flexible polymer dielectric barrier between all conductor layers and traditional electrical interconnection techniques. These technologies, due to the expensive materials used and the large number of process steps are prohibitively expensive. Existing flexible circuit process complexity and expense preclude the practicality of continuous multilayer manufacturing schemes.
These deficiencies and problems are overcome by the present invention.